Metal substrates for industrial use have heretofore been subjected to a phosphate treatment as a surface preparation for enhancing corrosion resistance and adhesion. In chemical conversion treatment using a phosphate treatment agent, however, the agent contains large amounts of phosphorous and nitrogen, and also contains large amounts of nickel, manganese and other heavy metals for enhancing the performance of the formed chemical conversion film. This leads to problems with respect to impact on the environment, as well as from the aspect of industrial waste treatment owing to the generation of large quantities of post-treatment sludge containing zinc phosphate, iron phosphate and the like.
Further, as shown in FIG. 1, in order to improve the corrosion resistance of metal substrates for industrial use, the coating line includes numerous steps such as “degreasing—surface conditioning and chemical conversion treatment—water washing—electrodeposition coating—UF washing—pure water washing—baking-drying,” thus requiring much space and time.
Japanese Unexamined Patent Publication No. 2003-155578 teaches a chemical conversion treatment agent for iron- and/or zinc-based substrates that contains zirconium ions and/or titanium ions as well as fluorine ions and is substantially free of phosphate ions. However, treatment using the chemical conversion treatment agent for iron- and/or zinc-based substrates taught by this publication has a drawback in that it must be followed by application of a coating because without such a coating it is impossible to achieve adequate corrosion resistance and finish.
WO 02/103080 teaches that use of a metal surface treatment composition including (I) a compound containing at least one metal element selected from among Ti, Zr, Hf and Si and (II) a fluorine-containing compound serving as a supply source of fluorine ions makes it possible to deposit a surface treatment coating excellent in corrosion resistance on a metal surface containing at least one of iron and zinc and further to eliminate the need for a surface conditioning step, thereby streamlining the treatment steps and saving space.
Japanese Unexamined Patent Publication No. 2003-253461 teaches a metal surface treatment composition for iron-based substrates that contains zirconium ions and/or titanium ions and fluorine ions, as well as soluble epoxy resin, is substantially free of phosphate ions, and has a pH of 2.5 to 4.5.
Japanese Unexamined Patent Publication No. 2005-2370 teaches a surface treatment method for aluminum-based substrates, the method comprising the steps of (1) forming a chemical conversion coating film on an aluminum-based substrate surface by the chemical conversion reaction of a chemical conversion agent comprising a fluorine- and zirconium-containing compound, and (2) forming a hydrophilic coating using a hydrophilizing agent, the method being characterized in that the chemical conversion reaction conducts chemical conversion by electrolysis.
However, WO 02/103080 and Japanese Unexamined Patent Publication Nos. 2003-253461 and 2005-2370 also have the problem of not being able to achieve adequate corrosion resistance and finish unless a coating is applied following the treatment with the surface treatment composition taught thereby.
Japanese Unexamined Patent Publication No. 2006-28543 teaches a precoating process characterized in that after subjecting the article to be processed to degreasing and washing, a chemical conversion coating solution is applied without conducting surface conditioning and thereafter electrodeposition coating is performed without carrying out industrial water washing or drying of the treated article, the process being further characterized in that the chemical conversion coating solution may contain zirconium ions. On pages 4 to 5 of the specification, it is stated that the low pH of the chemical conversion coating solution makes a “water washing step” necessary between the chemical conversion step and the electrodeposition coating step, for preventing rusting.
Japanese Unexamined Patent Publication No. 2006-28579 teaches a precoating process characterized in that the article to be processed is treated by immersing it in the chemical conversion coating solution in a chemical conversion treatment tank in which an oil-adsorbent resin mass is floated, the process being further characterized in that the chemical conversion coating solution may contain zirconium ions. Moreover, paragraph 0034 of the specification states that no “washing process” using industrial water is necessary following the chemical conversion step.
However, when the precoating process of Japanese Unexamined Patent Publication No. 2006-28579 employing a chemical conversion coating solution with a resin floated thereon is used to form a coating and electrodeposition coating is then carried out without water washing, the chemical conversion coating solution is carried into the electrodeposition coating composition tank, so that the finish and corrosion resistance of the obtained coating film are unsatisfactory. Improvement has therefore been desired.
Japanese Unexamined Patent Publication No. 2006-239622 teaches a coating method that comprises the step of applying a water-base coating composition capable of forming, on a metal substrate not subjected to chemical conversion treatment, a coating having performance comparable to that of a coating including a chemical conversion coating film, which water-base coating composition contains, as appropriate, a water-dispersible epoxy resin and/or an anticorrosion pigment having a solubility of 0.001 to 5% in water. However, the method does not provide sufficient corrosion resistance.
WO 2006/109862 teaches a method for forming a coating film using an aqueous coating composition containing rare earth metals (Ce, Y, Nd, Pr, Yb) and, as appropriate, copper and/or zinc, and also containing a cationic resin; in which method a first layer is deposited in a pretreatment step at not greater than 50 V and a second layer is deposited at 50 to 450 V; the use of the aqueous coating composition enabling integration of the pretreatment step and the electrodeposition coating step. However, the coating film obtained using the aqueous coating composition set out in WO 2006/109862 is insufficient in corrosion resistance and particularly in long-term corrosion resistance properties such as weathering corrosion resistance.